Thermocouple ribbon and assembly

ABSTRACT

A thermocouple ribbon features a pair of flat conductors and first and second layers of a polyimide film covering the conductors. The polyimide film preferably is coated with a fluoropolymer, such as fluorinated ethylene propylene (FEP). During manufacture of the thermocouple ribbon, the first and second layers of polyimide film, with the pair of flat conductors positioned there between, are heated above the melting temperature of the FEP. The completed thermocouple ribbon is then cooled. A thermocouple connector may then be attached to a first end of the thermocouple ribbon, while a welded thermocouple junction may be formed at a second end of the thermocouple ribbon.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation to U.S. patent application Ser. No.14/016,877 filed Sep. 3, 2013, entitled Thermocouple Ribbon andAssembly, which claims priority to U.S. Provisional Patent ApplicationNo. 61/695,865, filed Aug. 31, 2012, the contents of which are herebyincorporated by reference.

FIELD OF THE INVENTION

The present invention relates generally to thermocouple assemblies andconductors and, in particular, to a generally flat thermocouple ribbonhaving generally flat conductors and thermocouple assemblies fabricatedfrom such a ribbon.

BACKGROUND

Autoclaves are large ovens used to bake or “heat cure” aerospace partsmade out of carbon/fiber composites. Once the parts are “formed,” theyare installed on holding fixtures and are rolled into the autoclaves forbaking. There is also the situation of repairing composite parts thatcome from an airplane or from composite production lines. Before theseparts are placed on the holding fixtures, a vacuum bag is placed toencompass the part to be baked. During the curing process a vacuum ispulled.

In order to monitor temperature of the curing process, thermocouplecables are placed on the parts and the thermocouple cables travel out ofand under the vacuum bag. As vacuum is pulled, atmospheric pressure willpress the vacuum bags against the part being cured. At curingtemperatures, the part will get soft and the compression of the vacuumbag over conventional thermocouple cables will leave an indentation ormark-off, which is unacceptable.

In addition, vacuum seal integrity is required during the manufacturingprocess of carbon composite components for aircraft. A vacuum isnecessary to rid the layered composite assembly of air pockets whichcause structural weakness in the assembly, to compress the compositelayers to conform to the desired shape, and to increase thefiber-to-resin ration to optimize strength of the composite. Temperaturemonitoring thermocouples require a vacuum seal since the thermocouplejunction is in the vacuum side of the process and the temperaturemeasuring instrumentation is necessarily on the ambient atmosphere side.

Vacuum seals around thermocouples monitoring the temperature of thecomposite curing process are usually made with pliable compounds orgaskets. To facilitate a successful vacuum tight seal, the gasketmaterial conforms more easily to a flat profile thermocouple wire asopposed to the typical dumb-bell shaped wire pair profile. A needtherefore exists for such a cable that reduces or eliminates thepossibility of leak path through the space between the conductor andinsulating material.

There are several prior art ribbon cables offered by various sources.None of the prior art ribbon cables, however, are designed with uniquefeatures, such as those to address the issues described above, for usein autoclave applications.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of an embodiment of the thermocoupleribbon conductor of the invention;

FIG. 2 is a perspective view of a thermocouple ribbon conductor attachedto a thermocouple connector in an embodiment of the thermocoupleassembly of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

As illustrated in FIG. 1, an embodiment of the invention includes a flatthermocouple ribbon, indicated in general at 10, with flat conductors 14a and 14 b insulated by thin layers of fluorinated ethylene propylene(FEP) fluoropolymer coated polyimide film 16 a and 16 b. The flatconductors preferably include pairs of dissimilar alloys such as thosealloys used as thermocouple conductors known in the art. The inventionis also directed to thermocouple assemblies and devices fabricated fromthis flat thermocouple ribbon, an example of which is provided in FIG. 2as described below.

Embodiments of the invention are unique with regard to the followingfeatures: (a) Ultra-thin thermocouple ribbon—the preferably 0.012″ orless in thickness of the ribbon is four times thinner than equivalentround cables and the flat contour doesn't leave any significantindentations in parts during autoclave applications, and (b) Design andcontour prevent leak—due to the nature of the flat conductor (preferablyapproximately 0.004″ thick) and ribbon, and due to the unique materialused, the cable prevents leak (ingression of air into the autoclavevacuum).

Thermocouple wire and assemblies are used universally to measuretemperatures of devices and processes. Typically, the wires used inthese applications are round standard gauge wire used in pairs ofdissimilar alloys (those alloys used as thermocouple conductors known inthe art, for example). The thermocouple pairs are normally insulatedwith an extruded electrical insulating jacket of a common polymer—FEP,polytetrafluoroethylene (PTFE), silicone or polyvinyl chloride (PVC), toname a few. The extruding process requires a thick wall for the successof the extruding process. Embodiments of the invention significantlyreduce the thickness of thermocouple pairs and provides a nearly flatform factor over conventional extruded jacket thermocouple wire.

As indicated above, there are two features that are unique toembodiments of the invention:

Thin cross section—embodiments of the invention incorporate a “flatconductor,” for example, 0.004″ thick on 28 gauge. This thickness isequivalent to 36 gauge, a much smaller gauge and very problematic touse. The combination of a flat conductor and sandwiching of twopolyimide tapes helps to achieve the 0.012″ thickness. The embodimentsof the cable of the present invention, which are preferably 0.012″ orless in thickness, spread the force of compression, which eliminates orreduces indentations in parts. This aids in repair of thermally curedcomposite assemblies where the thermocouple is trimmed and remains inplace after the cure, and therefore requires a very small cross-sectionto maintain structural integrity of the original composite device, andis cosmetically acceptable in appearance after the repair is completed.

Vacuum seal integrity—Preferably a 0.004″ thick polymide (PI) film thathas 0.002″ layer of PI and 0.002″ thickness of FEP is used as theinsulation or covering of the conductor. PI film (or KAPTON availablefrom E. I. du Pont de Nemours and Company of Wilmington, Del.) iscommonly available only in thickness up to 0.002″ with FEP coatingthickness of 0.0002″. This unique tape gives a robust bonding of the twotapes in the manufacturing process. The higher amount of FEP, whenmelted, seals all possible leak paths.

Therefore, with reference to FIG. 1, in accordance with the invention,flat thermocouple ribbons are fabricated by laminating a pair offlattened thermocouple wires 14 a and 14 b between two thin layers ofpolyimide film (preferably 0.004″ each layer) 16 a and 16 b or similarmaterial. The polyimide material is pre-coated on one side with a thinlayer (preferably 0.002″) 18 of a low melting point polymer (FEP forexample). As a result, a sealed, laminated flat thermocouple pair isformed by arranging two parallel thermocouple wires between an upperlayer and lower layer of FEP coated polyimide. The assembly is passedthrough a heat source (hot air, linear furnace, or other source of heat)to increase the temperature of the FEP to its melting point. Uponcooling, the FEP layer solidifies and bonds the assembly together. Theresult is a ribbon-like, very flat thermocouple wire pair assembly thatis as thin as 0.012″ and preferably ¼″ wide (for a thermocouple with awire cross-section equivalent to 24 AWG wire—see FIG. 1). Variations ofthis assembly concept can be manufactured to other dimensions andequivalent AWG wire size, both larger and smaller.

Ready-to-use thermocouple assemblies are fabricated using thisthermocouple ribbon. These thermocouple assemblies consisting of aprecut length of the flat thermocouple ribbon and include a weldedthermocouple junction on one end of the ribbon and a thermocoupleconnector on the other. As illustrated in FIG. 2, there are manystandard thermocouple connectors 12 that are commonly used that can beattached to the ribbon 10, including those mechanically attached andthose injection molded to the ribbon wire.

While the preferred embodiments of the invention have been shown anddescribed, it will be apparent to those skilled in the art that changesand modifications may be made therein without departing from the spiritof the invention, the scope of which is defined by the appended claims.

The invention claimed is:
 1. A thermocouple ribbon comprising: a) a pairof flat conductors constructed from a pair of dissimilar alloys, whereinthe conductors are flat throughout their entire length, therebyminimizing an overall thickness of the thermocouple ribbon; b) an upperlayer of polyimide film laminating the pair of flat conductors; and c) alower layer of polyimide film laminating the pair of flat conductors,the upper layer and the lower layer of polyimide film each comprising alayer of polyimide and a layer of fluorinated ethylene propylenefluoropolymer, wherein the upper layer and the lower layer of polyimidefilm provide a vacuum seal integrity along the entire length of thethermocouple ribbon, wherein each layer of fluorinated ethylenepropylene fluoropolymer is a same thickness in both the upper layer andthe lower layer of polyimide film, and further wherein a thickness ofeach layer of polyimide is equivalent to the thickness of each layer offluorinated ethylene propylene fluoropolymer, wherein the vacuum sealintegrity is created by heating the upper layer and the lower layer ofthe polyimide film to increase the temperature of each of the layers offluorinated ethylene propylene fluoropolymer to a melting point of thefluorinated ethylene propylene fluoropolymer, and upon cooling, thelayers of fluorinated ethylene propylene fluoropolymer solidify and bondthe thermocouple ribbon together.
 2. The thermocouple ribbon of claim 1,wherein each layer of fluorinated ethylene propylene fluoropolymer is 2mils thick in both the upper layer and the lower layer of polyimidefilm.
 3. The thermocouple ribbon of claim 1 wherein each layer ofpolyimide is approximately 2 mils thick.
 4. The thermocouple ribbon ofclaim 1, wherein the conductors are approximately 4 mils thick.
 5. Thethermocouple ribbon of claim 1, wherein the overall thickness of thethermocouple ribbon is approximately 12 mils.
 6. The thermocouple ribbonof claim 1 wherein a width of the thermocouple ribbon is approximately0.25 inches or less.
 7. The thermocouple ribbon of claim 1, wherein athermocouple connector is attached to a first end of the thermocoupleribbon.
 8. The thermocouple ribbon of claim 1, wherein a weldedthermocouple junction is formed at a second end of the thermocoupleribbon.
 9. A thermocouple assembly comprising: a) a ribbon including: i.a pair of flat conductors constructed from a pair of dissimilar alloys,wherein the conductors are flat throughout their entire length, therebyminimizing an overall thickness of the ribbon; ii. an upper layer ofpolyimide film laminating the pair of flat conductors; and iii. a lowerlayer of polyimide film laminating the pair of flat conductors, theupper layer and the lower layer of polyimide film each comprising alayer of polyimide and a layer of fluorinated ethylene propylenefluoropolymer, wherein the upper layer and the lower layer of polyimidefilm provide a vacuum seal integrity along the entire length of theribbon, wherein each layer of fluorinated ethylene propylenefluoropolymer is a same thickness in both the upper layer and the lowerlayer of polyimide film, and further wherein a thickness of each layerof polyimide is equivalent to the thickness of each layer of fluorinatedethylene propylene fluoropolymer, wherein the vacuum seal integrity iscreated by heating the upper layer and the lower layer of the polyimidefilm to increase the temperature of each of the layers of fluorinatedethylene propylene fluoropolymer to a melting point of the fluorinatedethylene propylene fluoropolymer, and upon cooling, the layers offluorinated ethylene propylene fluoropolymer solidify and bond theribbon together; b) a thermocouple connector attached to a first end ofthe ribbon.
 10. The thermocouple assembly of claim 9 further comprisinga welded thermocouple junction at a second end of the ribbon.
 11. Thethermocouple assembly of claim 9, wherein each layer of fluorinatedethylene propylene fluoropolymer is 2 mils thick in both the upper layerand the lower layer of polyimide film.
 12. The thermocouple assembly ofclaim 9, wherein each layer of polyimide is approximately 2 mils thick.13. The thermocouple assembly of claim 9, wherein the conductors areapproximately 4 mils thick.
 14. The thermocouple assembly of claim 9,wherein the overall thickness of the ribbon is approximately 12 mils.15. The thermocouple assembly of claim 9, wherein a width of the ribbonis approximately 0.25 inches or less.